Unfortunately Stereo Mix (sometimes called "Wave" or "Stereomix") is not available for all sound cards.
If you do not see it you might however just have to enable it first. In order to do so, right click your
speaker icon in your systems tray bar and press on "Recording devices". If you can't see this option, open the start menu,
type "Sound", hit enter and select the Recording tab on top. Right click any device in this window and
enable "Show Disabled Devices". If you now see a disabled stereo mix device, simply right click it and press "Enable".
Restart LightBeat and select it as your audio source.
If it doesn't show up you might have to first install or update your audio drivers, or try the next step to alternatively install a virtual audio cable.
We will use the free VB-Audio Virtual Cable, however you can use whichever you prefer. This particular one is free for one virtual device, which is all we need. Just press the "Download" button on the linked page and unzip the downloaded file. Right click the file "VBCABLE_Setup_x64.exe" and press "Run as administrator". If it shows you an error message try "VBCABLE_Setup.exe" instead. Now just click through the installer. If it asks you for permissions to install allow it, as it has to install a custom driver to your system.
After installation and a potential reboot, open up the Sound window from step 1 again, but this time click on the "Playback" tab on top. Right click on the now installed device "CABLE Input" and select "Set as Default Device". Now select the "Recording" tab on the top of the window next to "Playback" and double click "CABLE Output". Press on the "Listen" tab, tick the "Listen to this device" box and in the list below, select your actual speakers. That's it. Now you only need to select "CABLE Output" as audio source in LightBeat and you should be good to go. To go back to your default settings, simply right click your speakers in the "Playback" tab and set them as default device again. If you are on Windows 10 you can left click on the speaker icon and directly select your default speakers there to easily toggle between them.
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